Image-guided surgery (e.g. image guided biopsy) provides a surgeon with images during a surgical procedure such that the procedure is accurate and minimally invasive. In image-guided surgery, the surgeon observes the images while the operation is being carried out. In general, pre-operative medical images are acquired and analyzed to make a decision whether and/or how to perform a procedure. Once in the operation room, medical images are acquired again as the procedure is initiated. The surgeon(s) relies on accurate correspondence between the pre-operative images and the images acquired during the surgery. Due to motions of the patient and motion of internal anatomy due to bodily functions as well as due to surgical procedure, the images acquired during surgery often do not correspond well with the pre-operative images. Image registration may be performed on the two images to put them in same co-ordinate system. Image registration provides a point-wise correspondence between a pair of anatomical images such that every point in one image has a corresponding point in the other image. Therefore, image registration is an important part of interventional surgery as well as image-guided biopsy.
Image guided surgery may be used in 3-D ultrasound guided breast (or prostate) biopsies for collecting samples of abnormal tissues. During the procedure, 3-D ultrasound breast images are obtained and tissues appearing abnormal on ultrasound images are identified. A surgical needle is inserted into body and is guided to the abnormal regions for collecting sample of the tissue in that region. The extracted tissue is later examined by the pathologist to determine whether the tissue is malignant or benign and also the type and extent of malignancy. Due to patient motion and dynamic nature of image acquisition, the images have to be registered in real-time to guide the needle to the correct co-ordinates.
Image registration also finds application in serial radiation therapy, where a patient is delivered radiation dosage for cancer treatment over a period of time. In such cases, the time between the previous scan/image and the current scan/image can be of the order of weeks and there are numerous differences between the images acquired at the two temporally separated instants. In such cases, there is a need to align the current image with the previous image(s) to find corresponding structures as well as to determine dosage.
Registration is also useful in removing motion artifacts from DSA images during surgical intervention. DSA images are acquired as a movie constructed from projection X-ray images while a contrast enhancement dye is injected into the blood stream. Frames from the movie are acquired before the contrast enhancement agent reaches the field of view, and after it has reached the blood vessels in the field of view. The injected dye results in a significant enhancement of blood vessels and a digital subtraction is typically performed between the images with the dye and without dye to remove the background structures such that only blood vessels are visible. The subtraction images are called DSA (Digital Subtraction Angiography) images and are further enhanced to aid the surgeons during the interventional procedure. Due to movement of the background structures, however, the background structures show up as artifacts in the subtraction images, and get further enhanced after intensity enhancement. If the images are registered together as it is acquired in real-time, then the DSA output will contain only the blood vessels in the DSA images by matching the background structures together.
All these applications require the registration of a current medical image to a pre-operative medical image to be substantially real-time. Due to complex movements of tissue structure, the transformation can not be approximated using a model having a small degree of freedom, such as rigid and affine registrations. The transformation needs to have a large degree of freedom and a non-rigid elastic registration is required to better approximate the transformation. However, the elastic registrations take a long time to register the images and the overhead in terms of time taken are prohibitively high for most techniques. If an extremely accurate registration algorithm takes a long time while the patient is in operation and surgeons are waiting, such a method may not have any practical use.